Frankia sp.
strains comprise a distinct lineage among the high Mol% G+C Gram positive Actinobacteria .
They are filamentous "euactinomycetes" that grow by hyphal branching
and tip extension and thus resemble the antibiotic-producing Streptomyces sp. Frankiae
are developmentally complex, producing three cell types during growth: vegetative
hyphae, sporangiospores and lipid-enveloped cellular structures known
as "vesicles". The latter develop during N-starvation and contain the
O2-labile nitrogenase. In symbiosis, they supply sufficient
combined nitrogen so that the plant can grow without added nitrogen.

Frankia sp.
strain HFPCcI3 was isolated in 1983 at Harvard Forest in John Torrey's
lab by Zhang Zhongze. HFPCcI3 grows slowly on organic acids
such as propionate or pyruvate and will grow on some complex media. It
is resistant to kanamycin and, like many actinomycetes, to nalidixic
acid. Its ultrastructure has been studied both with respect
to its free-living existence and its presence in nodules of Casuarina sp.
HFPCcI3 belongs to a group of closely related strains that nodulate Casuarina and Allocasuarina spp.
The Casuarinaceae is a tropical family of trees and shrubs whose natural
range includes Australia and the Melanesian region of the Pacific. They
have been planted on islands and in coastal regions of the Indian Ocean,
New Zealand, Africa and North and South America, especially in the
Caribbean islands, Florida and California where they have been used
for soil stabilization, as windbreaks and for lumber and firewood.

The Global
Importance of the Actinorhizal Symbiosis

The importance of Frankia sp.
strains derives from their ability to form nitrogen-fixing root nodule
symbioses on certain woody angiosperms, termed "actinorhizal plants".
Actinorhizal plants are classified in eight plant families, 24 genera
and more than 200 species. These symbioses add a large proportion
of new nitrogen to several ecosystems . They constitute
the major N 2 -fixing symbioses in temperate forests ( Alnus ),
dry chaparral and matorral ( Ceanothus, Trevoa, Talguenea, Chamaebatia,
Cercocarpus, Purshia ), coastal dunes ( Casuarina, Myrica, Hippophae ),
alpine communities ( Alnus ) and in colder regions ( Alnus,
Dryas ) such as in Scandinavia, Canada, Alaska or New Zealand where
legumes are insignificant or absent. Actinorhizal symbioses are thus
key to establishing and maintaining many forests and shrublands, particularly
on occasions where climatic or anthropogenic changes disrupt ecosystems.

For more information
see: http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm#